1. Field of the Invention
The present invention relates to a method for injecting fuel into a multi-cylinder internal combustion engine.
2. Description of the Related Art
When starting an Otto engine in the case of a known method, each of the cylinders is supplied once with what is referred to as a preliminary injection in order to wet the cylinder walls and, at the same time, provide an ignitable mixture for the initial combustion. Since there is as yet no synchronization between the camshaft and the crankshaft and the position of the pistons is unknown at this point in time, a selective preliminary injection strategy is required in order to minimize the emission of unburnt fuel and hence pollutant emissions during starting.
In a method disclosed by EP 0 317 158 B1, to which U.S. Pat. No. 4,998,522 corresponds, the cylinders are divided into a first and a second group of cylinders, depending on the two different levels of the camshaft signal. The cylinders of the first group are supplied simultaneously with the preliminary injections (group injections) immediately after detection of starting, while there is a delay between the preliminary injections for the cylinders of the second group. At the same time, the quantities of fuel chosen for the preliminary injections are the same.
It has now been found that the air charges in the cylinders are not the same, even when starting, due to the gas dynamics in the intake pipe. Thus, owing to the stationary column of air in the intake pipe the first cylinders are not so well filled as the subsequent cylinders, where the air in the intake pipe has already reached a significant flow velocity and a corresponding kinetic energy. The result is that the fuel/air mixtures (lambda values) of the initial preliminary charges are richer than the subsequent preliminary charges. This leads to increased pollutant emissions in the starting phase, something that should be avoided, especially in the case of internal combustion engines that are optimized in terms of pollutants.
The object on which the present invention is based is to specify a method for injecting fuel into a multi-cylinder internal combustion engine, in which fuel/air mixtures with different lamba values are largely avoided in the starting phase.
According to the invention, different quantities of fuel for the preliminary injections are specified, depending on the sequence of preliminary injections and on the air charges to be expected in the cylinders.
The present invention is based on the realization that, after being switched off with the clutch disengaged, an internal combustion engine always stops at certain discrete positions, the number of discrete positions over two revolutions of the crankshaft (760xc2x0) corresponding to the number of cylinders. In the case of n cylinders, there are thus n angular stopping positions, which are moreover at the same angular distances apart. Furthermore, tests have shown that the rotational-speed behavior of the internal combustion engine and the time relationship between the opening of the inlet valves and rotational speed during the starting phases is always similar, irrespective of the discrete position in which the internal combustion engine has stopped. Essentially the same sequence of different air charges for sequential preliminary injections is thus obtained with each start.
This makes it possible to estimate the air charges to be expected for the sequential preliminary injections. The quantities of fuel for the preliminary injections can therefore be chosen appropriately as a function of the sequence of preliminary injections and the air charges to be expected, it being necessary to determine the air charges to be expected only once and it then being possible to use the corresponding values for each start. Since the air charges depend primarily on the rotational speed in the respective intake phase, the air charges are preferably determined as a function of the rotational speeds to be expected in the respective intake phases.
In a particularly advantageous refinement of the method according to the invention, the quantities of fuel for the preliminary injections are determined by multiplying a standard quantity by weighting factors, which are each assigned to one preliminary injection. If the weighting factors could also be estimated, they are expediently determined experimentally for each series of an internal combustion engine and then stored in the central control unit.
As mentioned, the present invention exploits the fact that the internal combustion engine always stops at certain discrete positions after being switched off with the clutch disengaged. However, it should be emphasized that it is not necessary to know this stopping position to carry out the method according to the invention. On the contrary, it is sufficient for the method according to the invention to know the sequence of the preliminary injections in order to specify the quantities of fuel for the preliminary injections as a function of these.
By means of the method according to the invention, it is ensured that, even in the starting phase, the preliminary injections are dispensed in quantities that are at least approximately adequate for the respective air charge. Excessively rich or excessively lean fuel/air mixtures are thus avoided, resulting in a corresponding reduction in pollutant emissions.